The effect of glyphosate on aromatic amino acid metabolism in purple nutsedge sprouted tubers and shoots was investigated. Glyphosate at 33.5 mM caused inhibition of bud elongation, increased total free amino acid concentration, and caused rapid accumulation of shikimic acid in sprouted tubers. However, only one aromatic amino acid, tryptophan, decreased quickly to 22% of control 3 d after treatment (DAT) and remained low afterwards. This suggests that the inhibition of bud elongation is due to the rapid accumulation of shikimic acid and the repression of tryptophan synthesis. Foliar application of glyphosate at 14.5 mM to purple nutsedge shoots resulted in the rapid accumulation of glyphosate which was rapidly converted to its metabolite, aminomethylphosphoric acid. Free amino acids in leaves were also increased by glyphosate 3 DAT. The reduction in soluble protein 5 DAT and increased acid protease activity 3 DAT suggests that the late accumulation of free amino acids partially resulted from protein hydrolysis. Shikimic acid accumulated in glyphosate-treated leaves 5 DAT, but the concentration of the three aromatic amino acids was not reduced. This suggests that glyphosate toxicity in purple nutsedge shoots was associated with the rapid accumulation of glyphosate, followed by large accumulation of shikimic acid. Aromatic amino acids deficiency was apparently not a factor in toxicity.

In order to explore the physiological mechanism of paraquat resistance in tall fleabane, a widespread weed in Taiwan where resistance to this herbicide has been observed since 1980, the role of the antioxidative system was assessed. The susceptible (S) and resistant (R) biotypes of tall fleabane were distinguished clearly by the relative distribution frequency of injury index caused by 78 µM paraquat. Although malondialdehyde, an indicator for peroxidation damage to the plant, in the the R-biotype was not changed, in the S-biotype malondialdehyde increased within 2 h after treatment of 50 µM paraquat. Analysis of several antioxidants and pertinent enzymes revealed that ascorbate peroxidase activity was decreased by paraquat treatment in the S-biotype; and a lower basal level of ascorbate was present in the S-biotype as well. The maintenance of a high ratio of reduced glutathione to total glutathione, coupled with a pronounced and rapid increase of glutathione reductase (GR) activity in the the R-biotype, suggests that an active reduced glutathione/oxidized glutathione (GSH/GSSG) cycle is critical to paraquat resistance of tall fleabane. The decisive contribution of a functional GSH/GSSG cycle to paraquat resistance through an enhancement of GR activity in this weed was further confirmed by an experiment of exogenous application of ascorbate.

Butachlor is the most commonly used herbicide on paddy fields in Taiwan and throughout Southeast Asia. Since paddy fields provide habitat for pond breeding amphibians, we examined growth, development, time to metamorphosis, and survival of alpine cricket frog tadpoles (Fejervarya limnocharis) exposed to environmentally realistic concentrations of butachlor. We documented negative impacts of butachlor on survival, development, and time to metamorphosis, but not on tadpole growth. The 96 h LC(50) for tadpoles was 0.87 mg/l, much lower than the 4.8 mg/l recommended dosage for application to paddy fields. Even given the rapid breakdown of butachlor, tadpoles would be exposed to concentrations in excess of their 96 h LC(50) for an estimated 126 h. We also documented DNA damage (genotoxicity) in tadpoles exposed to butachlor at concentrations an order of magnitude less than the 4.8 mg/l recommended application rate. We did not find that butachlor depressed cholinesterase activity of tadpoles, unlike most organophosphorus insecticides. We conclude that butachlor is likely to have widespread negative impacts on amphibians occupying paddy fields with traditional herbicide application.

The physiological basis of glufosinate resistance for two resistant (R) rice mutants, lines 'R11-2' and 'R11-3', was studied. Seven days after the application of 0.54 mM glufosinate, two susceptible (S) lines, i.e., variety (var.) 'FSK' and its inbred line 'FSK-3', and a reference var. Tainung 67 (TNG 67) suffered severe injury, whereas the two R lines exhibited resistance. Dose-response analysis and survival rate 14 d after treatment with 1.5 mM glufosinate also supported this observation. A C-14-glufosinate experiment showed that more labeled herbicide was absorbed by leaves of R11-2 than S lines 48 h after treatment (HAT), but the partitioning of absorbed glufosinate to each part of the shoot did not differ between R and S lines. Although a higher degradation of glufosinate in R line R11-2 was found as compared with the two S lines, i.e., 46% vs. 38 to 40%, the actual concentration of glufosinate in R line was still higher than that in S lines. Foliar application of glufosinate resulted in less inhibition of in vivo activity of glutamine synthetase (GS; EC 6.3.1.2) as well as a lower accumulation of ammonium 24 HAT in R line than in S lines. Further kinetic study of GS showed that cytosolic GS in line R11-2, with a higher enzyme-inhibition constant (Ki) value to glufosinate, was less sensitive to the toxic action of this herbicide. Therefore, a higher metabolism of, and more important, a lower susceptibility of, the target protein GS to this herbicide are suggested to contribute significantly to glufosinate resistance in these rice lines

Out of 1,343 mutant lines of rice mutated by sodium azide from the parental Japonica-type variety ‘Tainung 67’ (TNG67), a paraquat-susceptible line 1192 and a paraquat-tolerant line 72 were selected using whole seedlings at the four-leaf stage and leaf segments at the tillering stage as test materials. Further selection from progenies of these two mutant lines yielded the susceptible 1192-11 (S) and tolerant 72-16 (T), which were studied herein. Chlorophyll fluorescence, electrolyte leakage, and lipid peroxidation were measured for leaf segments of rice following treatment with 0.1 mM paraquat. A comparison of these responses among the three rice lines (TNG67, 72-16, and 1192-11), revealed a higher tolerance to paraquat in the tolerant mutant line 72-16 and the parental variety TNG67 than in the susceptible mutant 1192-11. Analysis of the antioxidative system in paraquat-treated leaf segments showed that the reduced form of glutathione (GSH) and the ratio of GSH to total glutathione increased by 3.5-fold within 6 h after treatment (HAT) and up to 5-fold 9 HAT in the T line, as compared with the S line. In view of the high activities of both dehydroascorbate reductase (DHAR) and glutathione reductase (GR) in paraquat-treated leaves of TNG67 and the T line, the antioxidative effect of the ascorbate–glutathione cycle is hereby proposed to play an essential role in paraquat tolerance of rice. Pretreatment of rice segments with spermine enhanced DHAR and GR activities as well as paraquat tolerance of the S line. These results suggest that the activity of ascorbate–glutathione cycle induced by spermine is involved in rice tolerance to this herbicide. Although kinetics studies showed no significant difference among the three rice lines in paraquat inhibition of GR, a lower affinity of enzyme to substrate (Km) in TNG67 and the T line and a higher maximal reaction rate (Vmax) in the T line for the oxidized glutathione substrate (GSSG) were detected. These observations further implicate the importance of glutathione reductase in paraquat tolerance of rice